Thesymmetry imposed on the neutrino mass matrix in the flavor basis is known to be quite predictive. We integrate this very specific neutrino symmetry into a more general framework based on the supersymmetric SOð10Þ grand unified theory. As in several other models, the fermion mass spectrum is determined by Hermitian mass matrices resulting from the renormalizable Yukawa couplings of the 16plet of fermions with the Higgs fields transforming as 10, 126, and 120 representations of the SOð10Þ group. Thesymmetry is spontaneously broken through the 120-plet. Consequences of this scheme are considered for fermion masses using both a type-I and a type-II seesaw mechanism. This scenario is shown to lead to a generalized CP invariance of the mass matrices and vanishing CP violating phases if the Yukawa couplings are invariant under thesymmetry. Small explicit breaking of thesymmetry is then shown to provide a very good understanding of all of the fermion masses and mixing. Detailed fits to the fermion spectrum are presented in several scenarios. One obtains a very good fit to all observables in the context of the type-I seesaw mechanism, but the type-II seesaw model also provides a good description except for the overall scale of the neutrino masses. Three major predictions on the leptonic mixing parameters in the type-I seesaw case are (i) the atmospheric mixing angle l 23 close to maximal, (ii) l 13 close to the present upper bound, and (iii) a negative but very small Dirac CP violating phase in the neutrino oscillations.

Consequences of a specific class of two Higgs doublet models in which the Higgs induced tree level flavour changing neutral currents (FCNC) display minimal flavour violation (MFV) are considered. These FCNC are fixed in terms of the CKM matrix elements and the down quark masses. The minimal model in this category with only two Higgs doublets has no extra CP violating phases but such a phase can be induced by adding a complex singlet. Many of the theoretical predictions are similar to other MFV scenario. The FCNC contribute significantly to B meson mixing and CP violation. Detailed numerical analysis to determine the allowed Higgs contributions to neutral meson mixings and the CKM parametersρ,η in their presence is presented. The Higgs induced phase in the B 0 d,s −B 0 d,s transition amplitude M d,s 12 is predicted to be equal for the B d and the B s systems. There is a strong correlation between phases in M d,s 12 and |V ub |. A measurable CP violating phase φ s = −0.18 ± 0.08 is predicted on the basis of the observed phase φ d in the B d system if |V ub | is large and close to its value determined from the inclusive b decays.

The charged fermion mass matrices are invariant under U (1) 3 symmetry linked to the fermion number transformation. Under the condition that the definition of this symmetry in arbitrary weak basis does not depend upon Higgs parameters such as ratio of vacuum expectation values, a class of two Higgs doublet models (2HDM) can be identified in which tree level flavor changing neutral currents normally present in 2HDM are absent. However unlike the type I or type II Higgs doublet models, the charged Higgs couplings in these models contain additional flavor dependent CP violating phases. These phases can account for the recent hints of the beyond standard model CP violation in the B d and Bs mixing. In particular, there is a range of parameters in which new phases do not contribute to the K meson CP violation but give identical new physics phases in the B d and Bs meson mixing.

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The charged fermion mass matrices are always invariant under U (1) 3 symmetry linked to the fermion number transformation. A class of two Higgs doublet models (2HDM) can be identified by requiring that the definition of this symmetry in an arbitrary weak basis be independent of Higgs parameters such as the ratio of the Higgs vacuum expectation values. The tree level flavour changing neutral currents normally present in 2HDM are absent in this class of models but unlike the type I or type II Higgs doublet models, the charged Higgs couplings in these models contain additional flavour dependent CP violating phases. These phases can account for the recent hints of the beyond standard model CP violation in the B d and B s mixing. In particular, there is a range of parameters in which new phases do not contribute to the K meson CP violation but give identical new physics contribution to the B d and B s meson mixing. Specific model realizations of the above scenario are briefly discussed. * anjan@prl.res.in † bhavik@prl.res.in